Complement receptor type 2 (CR2 or CD21) is a key interface between innate and adaptive immunity by serving as the receptor for complement component C3d, as well as for C3 and fragments of C3 that contain the C3d domain or a portion thereof, including but not limited to C3dg, iC3b and C3b (D. T. Fearon and R. H. Carter, Annu Rev Immunol 13, 127-49 (1995); D. T. Fearon, Semin Immunol 10, 355-61 (1998)). C3d and other CR2-binding C3 fragments that contain C3d or a portion thereof are covalently attached to foreign antigens (such as invading microorganisms) through the action of the classical, alternative or lectin complement pathways (S. K. A. Law and K. B. M. Reid, Complement, D. Male, Ed., In Focus (Oxford, UK:IRL Press., ed. second edition, 1995)). C3d- or other CR2-binding C3fragment-bound antigens then greatly amplify B cell responses by binding to CR2 through these C3 fragments at the same time as engaging the B cell receptor (BCR) via the bound antigen (R. H. Carter and D. T. Fearon, Science 256, 105-7 (1992); J. C. Cambier, Biochem Soc Trans 25, 441-5 (1997)). The cross-linking of CR2 to the BCR by C3d, C3, or other CR2-binding fragments of C3 that contain C3d or a portion thereof greatly amplifies a signal transduction cascade through the CR2/CD19/CD81 co-activation complex (D. T. Fearon, 1995 ibid.; D. T. Fearon, 1998, ibid.; J. C. Cambier, 1997, ibid; A. K. Matsumoto, et al., J Exp Med 173, 55-64 (1991)).
Human CR2 is also the obligate receptor for the Epstein-Barr virus (EBV) through its interactions with the gp350/220 viral membrane protein (J. D. Fingeroth, et al., Proc Natl Acad Sci USA 81, 4510-4 (1984)). EBV causes infectious mononucleosis, and is associated with Burkitt's Lymphoma and several other lymphomas and non-lymphoid tumors (M. Okano, Acta Paediatr 87, 11-8 (1998)). In addition, human CR2 serves as a receptor for CD23 (J. P. Aubry et al., Nature 358, 505-7 (1992)) and is thus a receptor for at least three biologically important ligands. Using genetically manipulated mice and animal models, CR2 has been shown to be essential for the development of normal humoral immunity to T-dependent antigens (T. Hebell et al., Science 254, 102-5 (1991); J. M. Ahearn, et al., Immunity 4, 251-62 (1996); H. Molina, et al., Proc Natl Acad Sci USA 93, 3357-61 (1996)) as well as possibly play an important role in the maintenance of B cell self-tolerance and the development of autoimmunity (A. P. Prodeus, et al., Immunity 9, 721-31 (1998)). CR2 has also been shown to mediate the interaction of C3-bound HIV-1 as an immune complex with B cells in a fashion that promotes transfer of virus and infection of CD4 T cells (S. Moir, et al., J Exp Med 192, 637-46 (2000)). CR2 also mediates direct infection of CR2-expressing T cells or other CR2-expressing cell lineages that are bound by HIV-1 immune complexes containing C3, C3d or other CR2-binding C3 fragments (including, but not limited to, HIV-1 complexed with C3d).
Interactions with all three human CR2 ligands require the first two of 15 or 16 short consensus repeat (SCR) domains (C. A. Lowell, et al., J Exp Med 170, 1931-46 (1989); J. C. Carel et al., J Biol Chem 265, 12293-9 (1990)). SCR domains, like Ig domains, are found in many proteins from both complement and non-complement families, and mediate diverse biological functions (A. P. Wiles, et al., J Mol Biol 272, 253-65 (1997)). Several of the SCR proteins also serve as receptors for important human pathogens. For example, in addition to CR2, CD46 is a Measles Virus receptor (R. E. Dorig et al., Cell 75, 295-305 (1993)), and CD55 is an echovirus receptor (T. Ward, et al., EMBO J 13, 5070-4 (1994); J. M. Bergelson, et al., Proc Natl Acad Sci USA 91, 6245-9 (1994)). Previously determined structures of SCR proteins containing two or four SCR domains have revealed a conserved core structure but variable orientations between domains mediated in part by relatively short 3-8 amino acid inter-SCR linker peptides (A. P. Wiles, et al., 1997, ibid.; P. N. Barlow, et al., J Mol Biol 232, 268-84 (1993); J. M. Casasnovas et al., EMBO J 18, 2911-22 (1999); R. Schwarzenbacher, et al., EMBO J 18, 6228-39 (1999)). As one of the major functions of SCR domains is to mediate protein-protein (such as receptor-ligand) interactions, and at least two SCRs have been found to be required for these interactions, the relative angle and orientation unique to each SCR-containing protein is likely to contribute to both biologic diversity as well as specificity. However, the lack of a high-resolution structure of a receptor-ligand complex in this family has hindered the understanding of the molecular recognition mechanisms of this class of proteins. With regard to the structure of CR2 and the molecular interactions with its ligands, C3d and EBVgp350/220, variable results have been obtained using mutagenesis, monoclonal antibody, and peptide strategies (C. A. Lowell, et al., J Exp Med 170, 1931-46 (1989); D. R. Martin et al., J Exp Med 174, 1299-311 (1991); H. Molina, et al., J Biol Chem 266, 12173-9 (1991); H. Molina et al., J Immunol 153, 789-95 (1994); D. R. Martin et al., J Virol 68, 4716-26 (1994); H. Molina, et al., J Immunol 154, 5426-35 (1995)).
Therefore, there is a need in the art for a three dimensional structure of CR2 in order to better understand the molecular recognition mechanisms of the protein and to enable the identification and/or design of compounds that mimic, enhance, disrupt or compete with the interactions of CR2 with its ligands.